ABSTRACT The myosin essential (ELC) and regulatory (RLC) light chains play important roles in cardiac muscle contraction, yet their specific roles in regulating myosin motor function are not well understood. The scientific premise of this application regards the role of two myosin light chain (MLC) regulators that modulate myosin motor function in vivo and in vitro: (1) the long N-terminus of cardiac ELC (N-ELC), and (2) cardiac myosin RLC phosphorylation (P-RLC). Mouse models of cardiomyopathy (HCM, RCM and DCM) along with N-terminally truncated ELC-?43 mice will be studied to fully comprehend the mode of action of these two myosin regulators in controlling cardiac muscle contraction in health and disease. The study question addressed here is how N-ELC and P-RLC work at the molecular, myofilament and whole heart levels? Our central hypothesis is that MLC regulators function as molecular and/or energetic triggers controlling myosin?s power stroke, ATP utilization and force production in cardiac muscle. AIM 1: THE ROLE OF N-ELC AND P-RLC IN THE REGULATION OF MYOSIN MOTOR FUNCTION IN DIFFERENT MODELS OF CARDIOMYOPATHY. This aim will focus on the molecular triggers and MLC regulators of heart remodeling at the level of myosin molecules. Hypothesis: N-ELC uses a novel mechanism of step-size and step- frequency modulation to control cardiac myosin power output and facilitate actin-myosin interaction, and this process is regulated by P-RLC. We will investigate whether and how HCM/RCM/DCM/?43 mutations in MLC regulate myosin ATP-turnover rates and affect the super-relaxed (SRX)? relaxed (DRX) equilibrium in the heart. AIM 2: INTERROGATE MLC-REGULATED MYOSIN MOTOR FUNCTION AND HEMODYNAMIC, CONTRACTILE AND ENERGETIC RESPONSES OF THE HEART. Mechanistic studies of Aim 1 and MLC-dependent alterations in myosin motor function will be integrated with the hemodynamic, contractile and energetic responses of the heart in vivo. Hypothesis: HCM, RCM, DCM and ?43 hearts exhibit different demands for ATP to sustain their hemodynamic and contractile functions in vivo, thus differently affecting mitochondrial bioenergetics. AIM 3: ASSESS PHOSPHORYLATION OF MYOSIN RLC AND ELC AS A MOLECULAR MECHANISM TO MITIGATE THE PATHOLOGY OF HCM, RCM AND DCM. The cardiac SRX serves as a modulator of cardiac energy utilization, involved in decreasing metabolic rate (load) in both, the normally functioning myocardium and during times of stress, e.g. cardiomyopathy. Hypothesis: Phosphorylation of myosin RLC, and possibly ELC, play a potential protective role in cardiomyopathy disease that involve alterations of the SRX state and the phosphorylation- induced shift in the super-relaxed (SRX) ? disordered relaxed (DRX) equilibrium toward the DRX state in which myosin heads can readily interact with thin filaments and produce force.